The present invention relates to a multi-pulse type coding system and, more particularly, to a multi-pulse type coding system for coding a speech signal at a low bit rate (a low transmission rate).
There are two main methods for efficient coding of an input speech signal. One is a spectral coding method which codes a spectral structure of the speech signal, and the other is a waveform coding method which codes a waveform of the speech signal itself. The spectral coding method is capable of transforming a speech signal at a remarkably low bit rate, e.g., 4.8 Kb/s, but degrades the quality of a duplicate speech waveform. On the other hand, the waveform coding method is capable of realizing a duplicate speech signal of relatively higher quality. However, the coding bit rate according to the waveform coding method is generally higher than that by the spectral coding method.
In the waveform coding method, an input speech signal is whitized so as to improve coding efficiency. This whitizing operation performs flattening of a spectral structure of the speech signal. Information on the spectral speech structure is, otherwise, required for reproducing the speech signal. In the waveform coding method, generally speaking, the spectral structure of the speech signal is transmitted by utilizing the spectral coding method.
In the waveform coding method, when a whitized speech signal is coded, an amount of information after coding depends upon a degree of whitizing. That is, the higher the degree of whitizing, the less the amount of information necessary for coding the whitized speech signal.
Multi-pulse type coding is known as one of the more efficient waveform coding methods. In multi-pulse type coding, the spectral structure of the speech signal is expressed by a set of Linear Predictive Coding (LPC parameters. On the other hand, the whitized speech signal is additionally expressed by a plurality of excitation pulses (multi-pulses) featured by their amplitudes and their position during a frame period. Such multi-pulse type coding is disclosed in U.S. Pat. Nos. 4,282,405, No. 4,472,832 and No. 4,701,954, for example.
One subject in the multi-pulse type coding is to reduce an arithmetic amount necessary for searching the multi-pulses. As a solution for this subject, there is known a method of searching the multi-pulses through cross-correlation calculation. In this method, the search of the multi-pulses is performed by considering correlations between a filtered impulse response waveform derived from the LPC parameters and the speech signal. Therefore, it is necessary to determine LPC parameters in a period sufficiently exceeding a duration time of an impulse response. Accordingly, the LPC parameters have been conventionally updated every 20 msecs, for example.
In order to precisely express the spectral structure of a speech signal, it is empirically known that a shorter period, e.g., about 5 msecs is preferable for updating the LPC parameters. However, for the aforementioned reason, the updating period of the LPC parameters has to be set at about 20 msecs in the multi-pulse type coding, limiting the expressiveness of the spectral structure. As a result, the coding efficiency is limited to a coding bit rate of about 8 Kb/s to maintain the coding quality. Namely, when the multi-pulse type coding of a coding rate less than 8 Kb/s is applied, the coding quality cannot be retained but may be inferior to that by the spectral coding method.